1. Field of the Invention
The invention relates to optical resonators and in particular to semi-monolithic cavities for doubling the frequency of the resonant beam and to the fabrication of semi-monolithic cavities for this purpose.
2. Description of the Prior Art
Compact and efficient, continuous wave lasers with wavelength below 600 nm are generally commercially unavailable. To generate coherent radiation with wavelength below 600 nm several approaches to double the available laser frequency using second harmonic generation in a nonlinear crystal have been used. Unfortunately, the peak output power of continuous lasers is generally low for any process that requires a high electric field strength in a laser beam which is incident on a nonlinear crystal.
To enhance the second harmonic output power, one method is to place the nonlinear laser within the cavity of the laser. See, Bergqusit, H. Hemmati, and W. M. Itano, "High power Second Harmonic Generation of 257 nm Radiation in an External Resonant Cavity," Optics Communication, V.43, N.6, 437-442 (1982). In this approach, the high circulating power within the cavity passes repeatedly through the crystal. Since efficiency in the nonlinear process is quadractically dependent on the input power, this approach generates substantially higher output power than a single pass through the crystal. For some lasers, such as a semiconductor lasers, intra-cavity frequency doubling is either not possible or too cumbersome to implement. Also, in the standing wave cavities, spatial hole-burning of the waves within the cavity results in multiple mode operation and could lead to strong amplitude fluctuations in the second harmonic output.
An alternative approach of the prior art is to double the fundamental frequency of the laser in a resonant external cavity. In this approach, a nonlinear crystal is located in a cavity external to the laser's own cavity. A portion of the laser's output is injected into the external cavity through a partially reflecting mirror. When the cavity is maintained on resonance with the input laser beam, constructive interference within the external cavity generates high circulating power. See, Ashkin, G. D. Boyd, and J. M. Dziedzic, "Resonant Optical Second Harmonic generation and Mixing," IEEE Journal of Quantum Electronics, V. QE-1, N. 6, 109-122 (1966). Depending on the losses in the external cavity, the circulating power could be tens of times that of the input beam power. A resonantly enhanced field at the fundamental wavelength then efficiently converts to the second harmonic. For this power to be efficient, the frequencies of the laser in the external cavity must coincide. An external cavity can resonate only in a single longitudinal mode. With a multimode laser incident on the external cavity, only one mode can resonate. Thus, the power content for the remaining modes is not used. Therefore, for the most efficient process, the input laser beam has to be a single mode laser.
FIGS. 1a and 1b are simplified block diagrams that depict examples of a prior art discrete and monolithic resonant external frequency doubling cavities respectively. See, Godberg, M. K. Chun, I. N. Duling, and T. F. Carruthers, "Blue Light Generation by Nonlinear Mixing of Nd: YAG and GaAlAs Laser Emission in a KnbO3 Resonant Cavity, " V.56, N. 21, 2071-2073 (1990); Koziovsky, C. D. Nabors, R. L. Byer, "Efficient Second Harmonic Generation of Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO: LiNbO3 External Resonant Cavity," IEEE Journal of Quantum Electronics, V.24, N.6, 913-919 (1988); and Briger, H. Busener, A. Hese, F. Moers, and A. Renn, "Enhancement of Single Frequency SHG in a Passive Ring Resonator," Optics Communication, V.38, #5,6, 423-426 (1981). In FIG. 1a, the laser system, generally denoted by reference numeral 10, includes a nonlinear crystal 12 between a full reflection mirror 14 and a partially reflective mirror 16. Light from a separate laser 18 is incident on a beam splitter or partially silvered mirror 20. The beam 22 splits with a portion 36 going to a piezoelectric (PZT) driven mirror 24 and the other split portion is reflected into photodetector 26. The output of photodetector 26 is coupled to and amplified by a frequency lock-in amplifier 28, whose output in turn is coupled to a servocircuit 30. The output of servocircuit 30 is coupled in turn to a summing node 32. The other input to summing node 32 is coupled to an oscillator 34 operating at the resonant frequency, f, whose output is also coupled to and drives frequency lock-in amplifier 28. The output of summing node 32 in turn is then coupled to PZT driven mirror 24. The split portion 36 of light transmitted from laser 18 through mirror 20 is incident upon PZT driven mirror 24, is optoelectromechanically phase modulated and reflected as beam 38 to mirror 14. This phase modulated light is recirculated between opposing cavity mirrors 14 and 16 through crystal 12 with transmission occurring through mirror 16 as output beam 40. By this arrangement only single mode laser light at or near the fundamental resonant frequency, f, or its harmonics are able to constructively interfere in crystal 12.
The monolithic external resonant cavity as shown in FIG. 1b similarly includes external laser 18, producing a laser beam 20 which is directed to a shaped nonlinear crystal 42. Crystal 42 has curved facets 44 and 46 at its opposing ends and a planar interlying facet 48. Beam 22 is partially reflective at facet 46 to reflect a portion of the incident beam into photodetector 50, which in turn is coupled to an electro-optical feedback control circuit 52. Control circuit 52 in turn is then coupled to crystal 42 to drive crystal 42 on resonance. A portion of beam 54 within crystal 42 is then transmitted through partially reflective facet 44 as an output beam 56. The example of the discrete mirror resonant cavity in FIG. 1a employs two or more cavity mirrors and works well and is a straightforward approach. In two mirror cavities feedback from the cavity to laser generally has a deleterious affect on the mode stability of the laser unless an isolator is placed between the laser and the cavity. Moreover, the second harmonic output is generated in two directions. This necessitates the use of at least one isolator between the laser and the resonant cavity. The addition of an isolator is undesirable due to the additional space required, the additional loss of power and its cost. If three or more mirrors are used in the cavity in a ring geometry as shown in FIG. 1b, the beam travels in one direction only and the reflective light from the input-coupler does not coincide with the incident laser beam.
In the example of the monolithic external cavity in FIG. 1b, the cavity mirrors are polished and coated directly on facets 44, 46 and 48 of crystal 42. The facets of cavity 42 form the base of the mirrors. Monolithic cavities of this type have the advantage over the discrete cavity of a lower overall intra-cavity loss, no dispersion induced mismatch, mechanical stability, good frequency stability, compactness and lower overall cost. However, monolithic cavities such as shown in FIG. 1b suffer from certain disadvantages relative to the discrete cavities as shown in FIG. 1a. These disadvantages include the susceptibility for manufacturing errors, a loss of all degrees of freedom in cavity alignment, and difficulty or cost in obtaining nonlinear crystals with an area sufficiently large in size for the implementation of specific cavity designs. In addition, changes may be required in the cavity's length to accommodate locking at different resonant frequencies, and such implementations can be difficult.
Therefore, what is needed is a design capable of frequency doubling in a manner which has the advantages of both a discrete mirror resonant cavity and monolithic external resonant cavity frequency doubling, but which is not susceptible to the disadvantages of either.